[#83] client: Implement status library

Define base `Status` interface. Provide the functionality to distinguish
success and failure returns. Provide functionality to transport statuses
over NeoFS API V2 protocol. Support success `OK` and failure `INTERNAL`
returns.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

client/status/status_test.go

@@ -0,0 +1,35 @@
+package apistatus_test
+
+import (
+	"errors"
+	"testing"
+
+	apistatus "github.com/nspcc-dev/neofs-sdk-go/client/status"
+	"github.com/stretchr/testify/require"
+)
+
+func TestErrors(t *testing.T) {
+	t.Run("error source", func(t *testing.T) {
+		err := errors.New("some error")
+
+		st := apistatus.ErrToStatus(err)
+
+		success := apistatus.IsSuccessful(st)
+		require.False(t, success)
+
+		res := apistatus.ErrFromStatus(st)
+
+		require.Equal(t, err, res)
+	})
+
+	t.Run("non-error source", func(t *testing.T) {
+		var st apistatus.Status = "any non-error type"
+
+		success := apistatus.IsSuccessful(st)
+		require.True(t, success)
+
+		res := apistatus.ErrFromStatus(st)
+
+		require.Nil(t, res)
+	})
+}